The invention provides a process for the selective catalytic reduction of nitrogen oxides with ammonia in the lean-mix exhaust gas from a combustion process.
Nitrogen oxides which are produced in combustion processes are included among the main causes of acid rain and of the environmental damage associated therewith. Sources of release of nitrogen oxides into the environment are mainly the exhaust gases from motor vehicles and also the vent gases from combustion plants, in particular from oil-, gas- or coal-fired power stations or from stationary internal combustion engines and from industrial operations.
One feature of the exhaust gases from these processes is their high oxygen content which makes it difficult to reduce the nitrogen oxides present therein. The air ratio lambda (λ) is frequently used to characterise the oxygen content. This is the air/fuel ratio, normalised to stoichiometric ratios, of the air/fuel mixture with which the combustion process is operated. In the case of stoichiometric combustion, the air ratio is one. In the case of superstoichiometric combustion, the air ratio is greater than 1; the resulting exhaust gas has a lean-mix composition. In the opposite case, a rich-mix exhaust gas is referred to.
A process which has been used for some time to remove nitrogen oxides from such exhaust gases is so-called ‘selective catalytic reduction’ (SCR) with ammonia on a specially designed reduction catalyst. Suitable catalysts for this are described, for example, in the patents EP 0 367 025 B1 and EP 0 385 164 B1. They consist of a mixture of titanium oxide with oxides of tungsten, silicon, vanadium and others. Catalysts based on zeolites exchanged with copper and iron have also been disclosed. These catalysts display their optimum activity at temperatures between 300 and 500° C. and with a molar ratio between the reducing agent ammonia and the nitrogen oxides of 0.6 to 1.6. Depending on how the combustion process is managed, 60 to 90 vol. % of the nitrogen oxides present in the exhaust gases consists of nitrogen monoxide upstream of the catalyst.
To perform this process in motor vehicles, the ammonia required for selective catalytic reduction has to be supplied on-board the vehicle. As an alternative to environmentally harmful ammonia, a compound which reacts to give ammonia, such as for example urea, may also be used. The advantage of this process is based on the fact that operation of the engine can be optimised independently of exhaust gas treatment. However, the large-scale use of this process requires the construction of a costly urea infrastructure.
In order to avoid the construction of a urea supply, EP 0 773 354 A1 proposes producing the ammonia required for selective catalytic reduction on-board the vehicle, from the fuel which is also being supplied. For this purpose, the internal combustion engine is operated alternately with a lean-mix and a rich-mix air/fuel mixture. The exhaust gas formed in this way is passed over a three-way converter catalyst and a catalyst for selective catalytic reduction. During operation with the rich-mix air/fuel mixture, the nitrogen oxides present in the exhaust gas are reduced to ammonia on the three-way converter catalyst under the reducing conditions of the rich-mix exhaust gas. The ammonia being formed is stored by the SCR catalyst. During operation with lean-mix exhaust gas, the nitrogen oxides present in the exhaust gas pass through the three-way converter catalyst and are reduced to nitrogen and water on the SCR catalyst, with consumption of the previously stored ammonia.
DE 198 20 828 A1 describes a process in which the internal combustion engine is also operated alternately with a lean-mix and rich-mix air/fuel mixture. The exhaust gas treatment system contains three catalysts, wherein a nitrogen oxide storage catalyst is located in the exhaust gas section of the engine, upstream of the three-way converter catalyst in the process described above. During operation of the engine with a lean-mix air/fuel mixture, a considerable proportion of the nitrogen oxides present in the exhaust gas is stored on the storage catalyst, whereas the remaining proportion of the nitrogen oxides is reacted on the SCR catalyst, with consumption of the previously stored ammonia. During operation of the engine with a rich-mix air/fuel mixture, the nitrogen oxides stored on the storage catalyst are released and react on the downstream three-way converter catalyst to give ammonia, which is then stored on the SCR catalyst.
EP 0 861 972 A1 describes a variant of this process, wherein the ammonia required is also synthesised on-board the motor vehicle, with the aid of a three-way converter catalyst, from the nitrogen, oxides present in a rich-mix exhaust gas. To produce the rich-mix exhaust gas stream, some of the cylinders in the internal combustion engine are operated with a rich-mix air/fuel mixture and the, exhaust gas from these is passed over the three-way converter catalyst separately from the lean-mix exhaust gas from the remaining cylinders in order to synthesise ammonia.
One essential disadvantage of the last three processes is based on the intervention in engine management which is required and on the high light-off temperature of the catalysts. As a result of the requirement to alter the exhaust gas composition between rich-mix and lean-mix in a cyclic manner, in order to form ammonia, the optimisation potentials with regard to engine efficiency cannot be achieved. In addition, using this process it is possible to match the amount of ammonia produced to the actual amount required only with great difficulty. This applies in particular when the load conditions in the engine are changing rapidly.
DE 199 03 533 A1 describes another process for the selective catalytic reduction of nitrogen oxides in oxygen-containing exhaust gases. In this case, in addition to the lean-mix exhaust gas from the engine, a rich-mix gas stream is produced, independently of how the engine is operated, and this is treated in an electrical gas discharge plasma in order to form the ammonia required for the reduction process. This rich-mix exhaust gas stream can be produced, for example, by a separate burner which is operated with a substoichiometric air/fuel mixture and provides a nitrogen oxide-containing exhaust gas. The plasma catalytic ammonia synthesis proposed here is more effective, from an energy and equipment point of view, than the solution in accordance with the three processes mentioned above.
Although the process in DE 199 03 533 A1 dissociates the synthesis of ammonia from the exhaust gas in the internal combustion engine, this process also presents enormous problems in rapidly matching the production of ammonia to the amount required, for example when the load conditions are altering.